1. Field of the Invention
The present invention relates to a method and apparatus for the detection of a fault condition in a gas detecting apparatus containing one or more electrochemical gas sensors.
2. Description of Related Art
Potentially dangerous gas mixtures may be found in many work place environments. These dangers include the risk of fire or explosion from combustible gases, exposure to toxic gases and excessively high or low concentrations of oxygen.
These dangers are well known and gas detection instruments are available to detect a wide range of gases. These instruments typically contain one or more gas sensors, which give a proportional electrical response dependent upon the concentration of the gas to be detected. If the concentration exceeds allowed concentration limits, then the instrument will provide an alarm to warn nearby personnel, or it may activate other remedial actions, such as to increase the ventilation.
Gas detection instruments for safety applications are broadly divided into two groups. In the first group are portable instruments, which are designed to be hand held or worn by the user and provide personal monitoring. This group also includes transportable instruments which although not handheld, are easily moved from one location to the next. In the other group are fixed instruments, which are typically wall mounted, to provide area monitoring.
Oxygen and many of the commonly encountered toxic gases are usually detected with amperometric electrochemical gas sensors. A typical electrochemical sensor is usually constructed with two or more electrodes in contact with an electrolyte. The electrode is usually separated from the outside environment by a gas porous membrane, and other diffusion barriers. The gas to be detected enters the sensor and passes through the membrane to the working electrode, where it is either oxidized or reduced, or the rate of oxidation or reduction of the electrode or another species in electrolyte may be limited depending on the availability of the toxic gas. The resulting electrical current is proportional to the rate at which the gas is being consumed by the electrode; this type of electrochemical sensor is therefore known as an amperometric sensor. The output current is usually linearly proportional to the gas concentration, since the response is limited by the rate at which the gas to be detected can diffuse into the sensor.
The theory of operation and practical usage of electrochemical gas sensors has been discussed in detail by Chang et al (S. C. Chang, J. R. Stetter, C. S. Cha, xe2x80x9cAmperometric Gas Sensorsxe2x80x9d Talanta, (1993), 40, 461) and by Hobbs et al (B. S. Hobbs, A. D. S. Tantram, R. Chan-Henry in xe2x80x9cTechniques and Mechanisms in Gas Sensingxe2x80x9d, Ed. P. T. Mosely, J. Norris, D. E. Williams, (1991). In these sensors, the analyte gas diffuses into the sensor through a diffusion barrier to one of the electrodes, known as the working electrode. The electrons required for the oxidation or reduction of the gas flow through the external circuit to/from the counter electrode, where an equal magnitude reduction or oxidation reaction respectively occurs, and this flow of electrons constitutes an electric current, which provides the output signal. The potential of the working electrode is selected such that all the analyte gas which reaches the electrode is electrochemically oxidized or reduced. The nature of the response of the sensor to a toxic gas depends on both the design of the sensor and the nature of the gas. Some gases such as carbon monoxide (CO) and hydrogen (H2) are oxidized at the electrode, whereas other gases such as chlorine (Cl2), oxygen (O2) and nitrogen dioxide (NO2) are usually reduced in the sensor.
Oswin et al in U.S. Pat. Nos. 3,909,386, 3,992,267 and 3,824,167 describe a sensor for carbon monoxide and many variations of this basic design are known in the prior art. For most sensors, an external circuit (a potentiostat) controls the potential of the working electrode. In some sensors, such as galvanic oxygen sensors, the potential is generated by the oxidation of the counter electrode. A sensor of this latter type is known as a galvanic oxygen sensor, and descriptions have been provided by Lawson in U.S. Pat. No. 4,085,024, Tantram et al in U.S. Pat. Nos. 4,132,616 and 4,324,632, Culliname in U.S. Pat. No. 4,446,000, Bone et al in U.S. Pat. No. 4,810,352 and by Fujita et al in U.S. Pat. No. 4,495,051.
The output of most amperometric sensors is proportional to the gas concentration, and is described by the following equation:
I=nFCDxcex94
where I is the current (A), n is the number of electrons, F is the Faraday constant (9.648xc3x97104 C/mol), C is the gas concentration (mol/cm3), D is diffusion coefficient (cm2/s) and xcex94 represents the cumulative diffusion barrier that the gas must pass through to reach the working electrode. In principle, it is possible to measure all the diffusion barriers comprising the sensor and thereby calculate xcex94, and hence calculate the sensitivity of the sensor; for example, see P. R. Warburton, M. P. Pagano, R. Hoover, M. Logman, K. Crytzer, Y. J. Warburton, Analytical Chemistry (1998), 70, 998. However, this calculation is not practical in common practice, and instead the gas detection instrument is calibrated by exposure to a test gas of known concentration and the output of the instrument is adjusted to match the nominal concentration of the gas. This calibration is usually performed manually and it is typically a tedious process, especially if there are a larger number of instruments. Calibration is also an expensive procedure, both in terms of the cost of the test gases with certified compositions and in terms of the labor time, and associated record keeping.
Automatic calibration methods have been described in the prior art, for example, Stetter et al in U.S. Pat. No. 4,384,925, Hyer et al in U.S. Pat. No. 4,151,738, Hartwig et al in U.S. Pat. No. 5,239,492 and Melgaard in U.S. Pat. No. 4,116,612 describe methods for automatic calibration of a gas detection instrument in which calibration gases are automatically applied to the sensors under microprocessor control. For portable instruments, so called docking stations are now available, such as the DS1000 Docking station from Industrial Scientific Corporation, Oakdale Pa. 15107, which perform the calibration and record keeping automatically.
Though most sensor technologies are very reliable, as required for a safety application, electrochemical sensors do sometimes fail in service. While most electrochemical sensors do not have a fixed service life, some sensors, such as galvanic oxygen sensors are consumed during the oxygen detection reaction and so have a limited lifetime. Whereas calibration is usually only performed at fixed time intervals, for many safety applications it is common practice to xe2x80x9cbump testxe2x80x9d gas detection instruments more frequently to ensure that they are working correctly. The bump test typically involves application of a test gas mixture for enough time to activate the warning alarms and/or other modes of display that indicate that the instrument responded correctly to the gas. The bump test gas procedure is commonly quicker than a calibration, but it still involves the expense of both time and test gas mixtures.
The cost and time required for manually performing calibration or bump tests on gas detection instruments have provided an incentive for the development of test methods which can be performed automatically by the instrument without human intervention. The optimum function test for a sensor is exposure of the sensor to an analyte gas of known concentration and measurement of the sensor""s response. However, cost, size and complexity of the apparatus limit the ability to achieve this goal. Many simpler methods have been devised to measure the functional status of the sensor.
Electrochemical gas generators for testing gas sensors are well known in the prior art, and are described, for example, by Wolcott in U.S. Pat. No. 4,460,448 and by Rohrbacker et al in U.S. Pat. No. 5,395,501, and these generators can be applied to automated testing of sensors. Automated bump test methods have been devised in which the test gases are generated as needed, such as the electrochemical gas generators sold by Analytical Technology Inc. of Oaks, Pa. 19456 (8 Page Technical Information Sheet, titled A world of gases . . . A single transmitter) to provide test gas to automatically check the performance of gas detection instruments, and ensure that the sensors are responding within their specified limits. Finbow et al in U.S. Pat. No. 5,668,302 have incorporated an electrochemical gas generator within an electrochemical gas sensor, behind the diffusion barrier, to provide a means for automatic function testing of the gas detection instrument. Dodgson et al describe adding another electrode to an electrochemical cell to produce a test gas in PCT published application WO 98/25139. In addition, two methods for chemically generating bump test gases have been described by the Applicant in pending U.S. applications Ser. No. 08/891,235, now U.S. Pat. No. 6,098,523, and Ser. No. 09/282,661.
In another approach, Grambow et al in U.S. Pat. No. 4,321,113 calibrate the electrochemical sensors for oxidizable toxic gases such as carbon monoxide by changing the sensor potential such that oxygen is reduced at the working electrode. Since the atmospheric oxygen concentration is essentially constant and the oxygen reduction and the toxic gas oxidation currents both depend on the activity of the electrode, variations in the oxygen reduction current can be used to calibrate the sensors.
Methods have also been devised which can achieve calibration without prior knowledge of the gas concentration. One method is based on the application of Faraday""s law of electrolysis to a known volume of gas, described by Tantram et al in U.S. Pat. No. 4,829,809 and by Matthiesen in U.S. Pat. No. 4,833,909. Capetanopoulos, in U.S. Pat. No. 5,741,413, and Applicant in U.S. Pat. No. 6,055,840 have designed means to measure the gas concentration and hence calibrate a sensor by changing the diffusion barrier in the gas path.
Methods have also been developed for determining if the response of a sensor to gas is limited by diffusion, or if the reaction has become rate limited by the electrode kinetics. The electrode reaction(s) may become limiting if the working electrode becomes fouled or deactivated, the counter electrode becomes deactivated or excessively polarized or for three electrode sensors if the reference electrode potential drifts. For example, Bryan et al added two external electrodes to a polarographic oxygen sensor, as described in U.S. Pat. Nos. 4,900,422 and 5,098,547, to determine if fouling had affected the output. By applying a potential to the external electrodes sufficient to electrolyze the water, oxygen gas was produced adjacent to the sensor, thus providing means to test its functional status. In addition, a small AC signal was applied between one of the external electrodes and one of the sensor internal electrodes and the impedance of the sensor diffusion membrane was measured to determine if it had become blocked.
In another example, Wang et al in U.S. Pat. No. 5,558,752 describe a method wherein the potential of the working electrode is varied with respect to the counter electrode of a two electrode polarographic sensor to determine if the output current changes with potential. If the steady state output does not change with potential, then the response is limited by diffusion and not by the electrode kinetics. Clearly, this approach can also be applied to amperometric sensors with three or more electrodes, as has been described for example by Holmstrxc3x6m in PCT Published Application WO 99/22232. If the reference electrode potential drifts, then the electrode reaction may no longer be diffusion limited; this technique thus provides means to check for drift of the reference electrode. This method is applicable to both toxic gas sensors and oxygen sensors. Doer et al have also described electrical tests for the electrodes in HPLC electrochemical detectors in U.S. Pat. No. 5,100,530 that involve measuring the potential of the reference electrode versus the counter electrode. However, this test assumes that the counter electrode potential is steady, which may not always be a valid assumption. The reference electrode is usually designed to have a stable electrode potential, whereas stability of the counter electrode potential is not normally a design criterion for electrochemical detectors.
Other approaches to checking the functional status of electrochemical gas sensors have focused on the electrical properties of the sensor, such as the electrode capacitance. The electrodes used in many types of electrochemical gas sensors behave as though they have a large capacitance associated with them. The origin of this pseudo-capacitance is a combination of double layer capacitance and Faradaic processes occurring on the high surface electrode. The importance of electrode capacitance for electrochemical gas sensors has been discussed in more detail in P. R. Warburton, M. P. Pagano, R. Hoover, M. Logman, K. Crytzer, Y. J. Warburton, Analytical Chemistry (1998), 70, 998.
For a two-electrode cell, the capacitance of the whole cell is measured and it is not possible to separate the capacitive components of the working and counter electrodes. However, for a three electrode sensor, the capacitance of the working electrode can be measured independently of the counter electrode.
To a first approximation, an electrochemical sensor electrode can be modeled as a resistor and capacitor in series (RC circuit). Thus, if a potential step is applied to the sensor, there will be a current spike and subsequent current decay back to the steady state value. For a simple RC circuit, the current is described by the following equation:
I=(xcex94E/R)exp(xe2x88x92t/RC)
where I is the current, xcex94E is the change in potential, R is the resistance, C is the capacitance and t is the time from the potential step. In the prior art, potential step methods have been used to determine the functional status of the sensor. Jones in U.S. Pat. No. 5,202,637 applied a small voltage step to the sensor and monitored the resulting current spike. If the current spike exceeded a pre-determined limit, then the sensor was deemed to be working.
If instead of just recording the peak current on stepping the potential, the decay curve is recorded, then the electrode capacitance can be found. Capacitance C can be defined by the following equation:
C=xcex94Qxcex94E
where xcex94Q is the charge passed resulting from the change in potential xcex94E. The charge passed is most readily found by integrating the area under the curve and since the magnitude of the potential step is known, the electrode capacitance is readily found using the above equation.
Studer has described a similar potential step method in U.S. Pat. No. 5,611,909. If the current ratio is measured at potential step (It=0) and again at time t later (It=t), then the current ratio for a simple RC circuit is given by:
It=0/It=t=exp(xe2x88x92t/RC)
Studer used the ratio of the current values It=0/It=t to determine the functional status of the sensor. Studer defined two parameters Cm and Gm, which are characteristic of a sensor""s functional status. By comparison of these terms with the equation for a simple RC circuit, it is apparent that Cm is the electrode capacitance and Gm is the electrode conductance (inverse of resistance).
Makadmini and Horn in Transducers ""97, 1997 International Conference on Solid State Sensors and Actuators, Chicago, June 1997, Vol. 1, Paper 2A1.03, pp. 299 to 302, showed that the capacitance of a carbon monoxide sensor working electrode could be related to the active surface area of the electrode which in turn determined the sensitivity of that sensor. In a further development of this work, Makadmini et al in PCT Published Application WO 99/18430 varied an applied frequency to the sensor and monitored the magnitude and phase angle of the output signal. This method is a variation of electrochemical impedance spectroscopy EIS, which can be used to characterize the resistive and capacitive components of the impedance of a sensor. EIS is a very good method for measuring the status of an electrochemical sensor and diagnosing potential or actual faults, since small variations in the sensor electrodes often result in measurable changes in the EIS spectrum.
The use of electrochemical impedance techniques has also been described by Tomantschger in PCT Published Application WO 00/14523 to characterize and identify problems in potentiometric sensors used for measuring the concentrations of minor elements in molten metals.
Electrode capacitance can also be measured by cyclic voltammetry. This well known technique involves scanning the electrode potential E at a fixed scan rate (dE/dt) and measuring the current i. In the absence of an electroactive gas, the capacitance at each potential (so called differential capacitance) is given by the equation:
xe2x80x83dC=i/(dE/dt)
This method of measuring electrode capacitance is well known in the art of electrochemistry, and details can be found in standard text books, for example xe2x80x9cInstrumental Methods in Electrochemistryxe2x80x9d, by the Southampton Electrochemistry Group, Ellis Horwood Ltd, Chichester, 1985. Obviously, this method is readily applicable to toxic gas sensors, which typically exhibit a very large pseudo-capacitance associated with electrode redox surface processes in addition to double layer effects. For a toxic gas sensor, a small potential scan around the operating potential is preferred so as to minimize the time required for the sensor to be operable again after the test.
While certainly providing a simple and in-situ test that an instrument or controller can automatically perform on the sensor, these methods will only detect those modes of sensor failure, which affect the electrical properties of the sensor that these tests measure. For galvanic oxygen sensors, several methods have been described to predict the end of life of the sensor due to complete consumption of the anode. Tantram et al in U.S. Pat. No. 4,132,616 included a small amount of a second metal, such as copper, within the anode, which is more electropositive than the lead, but which is still sufficiently electronegative to provide the potential to the cathode for the reduction of oxygen. Once all of the lead has been consumed, the open circuit potential of the sensor will differ between lead as the active anode material and copper as the active anode material. Parker in U.S. Pat. No. 5,405,512 used multiple anodes of differing size, such that the earlier failure of the smaller anodes provided the end of life warning. Applicant has also described an electrical method that predicts the imminent failure of a galvanic oxygen sensor in U.S. application Ser. No. 09/135,058, now U.S. Pat. No. 6,096,186.
In a three-electrode sensor, the potential of the counter electrode is varied by the potentiostat circuit so as to maintain the potential difference between the working electrode and the reference electrode as a predetermined value. Further details of potentiostats can be found in standard text books on Electrochemistry, for example, the book xe2x80x9cElectrochemistry, Calculations, Simulation and Instrumentationxe2x80x9d, edited by J. S. Mattson, H. B. Mark Jr., and H. C. MacDonald Jr.; Marcel Dekker Inc, New York 1972. The potential of the counter electrode will vary from sensor to sensor, on applying gas and in response to anything else that acts so as to change the potential difference between the working and reference electrodes. Thus, measuring the potential of the counter electrode provides a means for determining the functional state of the sensor, making it possible to diagnose such conditions as broken leads to the reference or counter electrodes, and dry-out of the sensor.
Many manufacturers of gas detection instruments incorporate a resistor, EEPROM or other electronic device on the electrochemical sensor to provide means for the instrument to confirm the presence of a sensor and to identify the sensor type. This method is used, for example, in the WorksAlone II instrument from Industrial Scientific Corporation, Oakdale, Pa. 15071.
However, this method does not detect if there is electrical continuity between the electrodes. Broken contact wires between the electrodes and the external contact is one of the failure modes encountered with electrochemical sensors. Incorporation of means to measure the electrical conductance into the gas detection circuit provides a means for checking for an open circuit condition. The conductance should be measured with an alternating current or voltage signal to avoid polarization of the electrodes. Circuits needed to measure the conductance of an electrochemical cell are well known in the art of electronics, and are generally similar to those used in conductivity meters, for example, those from YSI Inc, Yellow Springs, Ohio 45387. Conductivity has been used to detect the functional status of electrochemical gas sensors; for example, Applicant described the use of conductivity to measure the status of galvanic oxygen sensors in U.S. patent application Ser. No. 09/135,058, now U.S. Pat. No. 6,096,186. Similarly, conductivity can also be used to check the functional status of electrochemical sensors for toxic gases.
Noise signals have been used in the prior art to ascertain the status of electrochemical gas sensors. Lindsay in U.S. Pat. No. 6,049,283 describes a method of checking the functional status of a sensor by monitoring the electrical noise produced by the sensor. If the root mean squared (rms) value of the noise is less than a predetermined threshold, the sensor is deemed to have failed. In another application, Tomantschger et al in PCT Published Application WO 00/14523 monitor the noise from an electrochemical sensor used for liquid metal analysis; if the noise levels exceed a threshold value, then the sensor is deemed to have failed.
While measuring the response of the sensor upon exposure to a known test gas is the most reliable method of ascertaining the current performance of a gas sensor, diagnostic tests offer several advantages. The prior art has shown that diagnostic tests may be able to provide a measure of the sensor function in situations where either a test gas is not available or where the test gas is available but the concentration is not known. Diagnostic tests in some cases also have the advantage that predictions of future failure of the sensor can be made while the sensor is still operational; however this approach has not been developed in the prior art. Indeed, tracking of the diagnostic results with time offers considerable potential to provide predictive capability. Furthermore, it may be advantageous to apply diagnostic tests in conjunction with the gas test to determine additional diagnostic information that is not available from either the gas test or the diagnostic tests individually.
It is therefore an object of the invention to use a modified potentiostat circuit for performing diagnostic tests on electrochemical gas sensors, allowing a gas detection instrument to perform routine tests on a sensor in addition to or in place of calibration.
It is a further object of the invention to use diagnostic tests to track the performance of a sensor and so predict the failure of the sensor in advance.
In a first embodiment of the invention, a voltmeter is included in the potentiostat circuit and is used to measure the potential difference between the reference electrode of the sensor and a metal coating on the outside surface of the sensor or on inside surfaces of the sensor that are not normally in contact with the electrolyte. In the event of an electrolyte leak, this metal coating will be contacted by the electrolyte and the metal coating will develop a redox potential. The voltmeter in the potentiostat circuit monitors the potential of the metal coating and allows a warning of an electrolyte leak.
In another embodiment of this invention, the potentiostat circuit is used to obtain the electrical time constant of the sensor by measuring the electrical noise in the sensor. The electrodes used in electrochemical gas sensors have a capacitance associated with them and the time constant acts as an RC filter to remove the higher frequencies from the noise. The frequency and amplitude distribution of the noise can be measured and the sensor time constant determined from the frequency cut off above which the noise signal is attenuated.
In another embodiment of this invention, the potentiostat circuit is used to obtain the electrical time constant of the sensor by measuring the response of the sensor to a single or double potential step waveform over a small potential range. From this test, the polarization resistance and the equilibrium potential of the working electrode are determined. Both of these parameters are indicative of changes in the electrode surface and provide markers for potential problems such as reference electrode drift or electrode poisoning.
In another embodiment of this invention, the potentiostat circuit is modified to allow the counter electrode of the sensor to be briefly disconnected from the circuit. During exposure of the sensor to the gas, the change in potential between the working electrode and the reference electrode during the period the counter electrode is disconnected is measured. From this result, the capacitance of the sensor can be determined and an analysis performed to determine whether the sensor response is limited by the rate of gas diffusion into the sensor.
In a further embodiment of this invention, a bipotentiostat is used to monitor the response current from the working and auxiliary electrodes of a sensor with four or more electrodes. If the both of the electrodes give a response proportional to the concentration of a test gas, then the ratio of the response from the working and auxiliary electrodes will be independent of the gas concentration. A change in the value of this ratio is indicative of a change in the sensor, and thus this ratio provides means for testing the functional status of a sensor.
In a still further embodiment of this invention, a potentiostat circuit is modified to allow the sensor to be tested galvanostatically. The use of small current flows through the sensor for short time periods allows the electrode capacitance to be determined. Passing larger currents through the sensor, and especially by varying the current passed with time, provides the means to characterize the electrochemical properties of the sensor. Comparison of these electrical properties with reference values or with data obtained at a different time can be used to determine the functional status of the sensor.